1. Field of the Invention
The present invention relates to a local antenna for the generation and/or for the reception of a circular magnetic high-frequency field in a nuclear magnetic resonance imaging apparatus with two oppositely disposed coil groups formed by two coils, the central coil axes of which are arranged at an angle to one another, in which for the generation of the circular magnetic high-frequency field the magnetic fields generated by the coils of a coil group are phase-shifted relative to each other by 90.degree. and in which for the reception of the circular magnetic high-frequency field the signals induced in the coil group, after a 90.degree. phase shift relative to each other, are superimposed.
2. Description of the Prior Art
In nuclear magnetic resonance imaging apparatuses, either linearly or circularly polarized magnetic high-frequency fields are used for the excitation of the nuclei. A linear field may be thought of as composed to two fields of equal magnitude respectively circularly polarized in opposite directions. This means that in a linearly polarizing antenna theoretically twice as much power is needed to achieve the same excitation as in a circularly polarizing antenna. Regulations for the protection of the patient permit only a maximal transmitting power. This restricts the possible sequences, especially the number of scanned anatomical layers in a given examination time. For this reason, with circularly polarized antennas theoretically twice as many (in practice not entirely twice as many) layers can be scanned in the same time as with a linearly polarizing antenna. The received signals then, after a 90.degree. phase shift of one signal, can be added in-phase. Thereby the useful signal is doubled, while the effective value of the noise is increased by the factor of 2. A circularly polarizing reception antenna thus has, with respect to a linearly polarizing reception antenna, a theoretical signal-to-noise gain of 2. The signal-to noise gain in practice, however, does not quite reach the theoretical value. Thus, both in transmission and in the reception, circularly polarizing antennas have advantages.
A so-called whole-body antenna or a body resonator designed for the generation and for the reception of a circularly polarized high-frequency field is disclosed in DE-OS 31 33 432. Two oppositely directed coil groups are arranged on an imaginary cylindrical surface, in which the central coil axes of the coils in a coil group are perpendicular to one another.
For the generation of a circular magnetic high-frequency field the coils of a coil group are fed with high-frequency currents phase-shifted relative to each other by 90.degree.. A patient to be examined is placed, for the production of tomograms, in the longitudinal direction within the cylindrical coil arrangement, so that the individual coils are located above and below as well as on both sides of the patient.
Similar to the above-described whole-body antenna, a local antenna for the examination of the head is known, with which circular magnetic high-frequency fields can also be generated. In contrast to the whole-body antenna, the head antenna can be opened in the lengthwise direction of the cylinder. After placement of the head on a lower part of the antenna, for the examination an upper part of the antenna is placed over the head and the head antenna is closed. Thus, with this antenna arrangement the body part to be examined is also completely enclosed by the antenna.
If section images of the upper body, and especially of the heart region, are recorded, with a conventional whole-body antenna, the whole-body antenna, because of its large reception surface, also receives noise signals from the other body parts. The signal-to-noise ratio is therewith worsened in such examinations. For this reason, for examinations of the upper part of the body, linearly polarized antennas are also used, although their effective field constituent is less than that of circularly polarized antennas.